Document expected `apiVersion` values for all API resources

[matlec]

Bug description

The spec uses apiVersion as a required field in request bodies (e.g., Device Client Onboarding), but the table only states:

apiVersion - Identifier of the version the API resource follows.

This doesn't specify what the actual value should be. The example shows "onboarding.margo.org/v1alpha1", but this is never explicitly documented as the expected value.

Issues:

1. Implementers don't know what value to validate against
2. Clients don't know what value to send
3. The pattern/convention for apiVersion strings is undefined (e.g., {resource}.margo.org/{version})

Proposed fix

For each API resource, explicitly document:

• The expected apiVersion value (e.g., onboarding.margo.org/v1alpha1)
• Whether servers should reject unknown versions or be lenient
• The versioning convention (e.g., v1alpha1 --> v1beta1 --> v1)

Anything else (optional)

No response